In an OFDMA communication system, the time-frequency resources of the system are shared among a plurality of mobile stations. Since different mobile stations have different channel conditions, quality of service (QoS) requirements, and capabilities, in some OFDMA communication systems, the time-frequency resources are divided into multiple regions to facilitate different types of transmissions. For time division duplex (TDD) systems, the time domain is divided into a downlink (DL) region and an uplink (UL) region. In some systems, the DL region and UL region are further divided into additional regions. For example, the DL may be divided into a partial usage of subcarriers (PUSC) region and a full usage of subcarriers (FUSC) region such as described by the IEEE 802.16 standard. Mobile stations assigned to the DL PUSC region experience less interference than mobile stations assigned to the DL FUSC region. Therefore, the DL PUSC region is often advantageous for mobile stations near the cell edge. The DL FUSC region utilizes the entire bandwidth in each sector, thereby maximizing the spectral efficiency. The DL FUSC region is advantageous for those mobile stations that can tolerate increased interference relative to what would be seen in the DL PUSC region and is therefore advantageous for mobile stations near the base station.
The base station assigns resources to mobile stations using an assignment message, which is transmitted as part of a control channel. The assignment message typically contains an indication of the assigned channel (channel identifier) and other parameters related to the transmission of a particular packet or series of packets. If multiple regions exist, it is known for the assignment message to explicitly indicate the region, either through additional bits in the overhead message or by embedding the region information in the channel identifier. An indication of the region, whether explicitly indicated or included as part of the channel identifier, creates additional control channel overhead. For wireless systems, it is essential that control channel overhead be carefully managed. Thus, there is a need for assigning time-frequency resources in a system with multiple regions without indicating the region.